1. Field of the Invention
The invention relates to setting a measurement aim region in a displacement sensor, and in particular, to a setting method for a measurement region having plural areas as aims each different in refractive index from the others, in a case where a transparent object such as glass is as a measurement aim object.
2. Description of the Related Art
In a non-contact displacement sensor (adopting a laser or the like as a light source) using a triangular distance measuring method, a light projecting element illuminates a measurement aim object with light directed thereto and information on which position of a light receiving element light reflected from the measurement aim object is converged to is employed to thereby measure a distance to the measurement aim object.
In a case where a transparent object such as glass is measured as a measurement aim with such a displacement sensor, however, two kinds of reflected light which are generated from a front surface and back surface of the glass are usually generated. Such a glass often has a back surface with a film such as a metal coat or the like even though a front surface has no coverage, and in a case where such a glass with a film is measured, a difference in reflected light quantity occurs between the front and back surfaces due to difference in refractive index. Examples thereof are a glass plate used as a Brown tube of a television receiver, a glass plate of a liquid crystal panel or the like. In such a case, if an emitted light quantity of the light projecting element and a light receiving sensitivity such as a light receiving gain of the light receiving element are adjusted in order to be adapted for one surface, a light receiving state on the other surface cannot be held to be proper (a received light quantity is excessively large or small), which makes positions of the front and back surfaces hard to be correctly acquired.
Besides, in a case where a glass measurement is conducted, there are incoming light by reflection from the front and back surfaces or a multiple reflection. In such a situation, a necessity arises for individual surfaces to be separately recognized with respect to light receiving positions in order to stably measure a surface displacement of the glass or a thickness of the glass. As a measure therefor, an area in the vicinity of a light receiving position which is desired to be measured is set as a measurement aim region, thereby enabling the set measurement aim region to be stably measured.
In a case where, as shown in FIG. 11A, in setting of a predetermined laser light emitting quantity and a light receiving sensitivity, a glass plate is measured as a measurement aim object, reflected light quantities from the front surface and back surface of a glass are clearly different from each other with respect to an image obtained from the receiving light element, leading to a chance of disabling either of both quantities to be measured. In setting of a predetermined laser light emitting quantity and a predetermined light receiving sensitivity, as shown in FIG. 11B, there often occurs a case where a received light image of only one of the front surface and back surface is obtained due to an excessively large difference in reflectance between the front and back surfaces of the glass. In such a case, if a laser light emitting quantity is increased or a light receiving sensitivity of the entire pixel region of a light receiving element is enhanced, so that stable measurement is secured for one surface from which a smaller received light quantity is given, saturation occurs for the other reflecting surface, having produced a problem that stable measurement is disabled.
In order to cope with such a situation, for example, a measurement region is, as shown in FIG. 11C, limited to a region enclosed by dotted lines, and a laser light emitting quantity or a light receiving gain is adjusted only for the region to thereby enable light from a surface on which measurement is desired to be stably received without exerting an adverse influence on the other surface.
A displacement sensor has been known that enables plural regions having respective arbitrary sizes to be discretely set in the entire pixel region of an image pick-up element (refer to, for example, WO 2001/057471).
In a case where a received light quantity from a surface with a smaller reflectance cannot be, as shown in FIG. 11(b), observed on a screen image obtained from a light receiving element because of a setting state of a light receiving sensitivity at that time, there is a high possibility that a sensitivity matching a surface from which a larger received light quantity given is set while not taking notice of the presence of the surface with a smaller reflectance. Though it is desirable to detect all the reflecting surfaces and to set the measurement aim regions on the respective surfaces, it can be said to be difficult to take notice of even a necessity for such an operation. In a case where reflecting surfaces are remote from each other and an image on one reflecting surface resides outside the measurable range of displacement measurement on a light receiving element, the image is not on the light receiving element; therefore, the reflecting surface cannot be found despite of all the effort of searching therefor and a measurement aim region cannot be set thereon. In such a case, operator in charge has a chance to regard it due to unskilled setting of a sensitivity despite the fact that the problem is caused by a distance between a glass, which is a detection aim, and a displacement sensor to repeat a try-and-error approach and to thereby increase time and labor to be consumed thereon. Since, in such a way, an image of a surface with a low reflectance has a possibility of the presence, even if the image is not observed and a case arises where a surface, which is low in reflectance as a measurement aim, cannot appear on an image of a light receiving element because of being outside the range of displacement measurement, an operation of setting a measurement region is of difficult determination having to give a consideration to a range of a displacement sensor, a spacing between reflecting surfaces of a measurement aim object and an installment spacing between the displacement sensor and a reflecting surface in addition to a necessity therefor, which is hard to take the operation to be performed by the operator at a job site with ease.
A problem has been pointed out that a user is necessary to be participated in such an operation, which loses an operational efficiency.
In a case where, in such a way, an approximate position of a measurement aim region is visually recognized by a user with a received light waveform displayed on a monitor or the like and thereafter a measurement region is set by manual input, a measurement region can be set in the situation shown in FIG. 11A, grasping with some accuracy which position a second reflecting surface is present at, while since it is not known where a second reflecting surface is present in a situation shown in FIG. 11B, a complicated operation is requested, in order to recognize the position, that a laser light emitting quantity of a light projecting element and a light receiving sensitivity of a light receiving element are adjusted in advance of setting of a measurement region, which have pointed a problem of a great reduction in operational efficiency.